Previous results have indicated very little NEM-HMM is bound to actin during ATP hydrolysis under conditions of maximum actin activation. To determine whether the same behavior would be shown by subfragment-1 which in contrast to HMM has only a single head we propose to use the analytical ultracentrifuge to investigate the binding of NEM- subfragment 1 to actin both in the presence and absence of salt very near to Vmax. We also plan to use the analytical ultracentrifuge to investigate the binding of actin to HMM which has been modified with NEM both at the SH1 and SH2 sites.